The present invention relates to improvements in a pneumatic pressure belt filter for dewatering fine coal including a method and mechanism utilizing a filter belt which intermittently passes through a pressure tank having movable sealing devices at the entry and exit from the tank.
In the art of pneumatic pressure belt filters, the filtrate chamber and the slurry chamber are frequently disposed in a yoke-like frame, and at least one of the two chambers may be pressed against the sealing surfaces of the other chamber over mechanical or hydraulic adjustment elements. A relatively complex filter means is involved.
One approach to simplify the structure and process is shown and described in German OS No. 20 62 345 wherein the filtrate chamber and slurry chamber are interconnected over an axis so that they can be angled or pitched upwardly with the axle serving as the axis of rotation. This mechanism is still relatively complex, particularly at high pressures and requires a substantial wall thickness, reinforcing ribs and so forth which lead to a very heavy construction.
An object of the present invention is to provide a pressure belt filter which is designed to be constructed in a uniquely simple and light manner, but which is capable of being operated at high pressures.
A feature of the invention is achieving the foregoing objective by constructing the pressure tank of a pressure belt filter as a cylindrical horizontal chamber which has rectangular openings for the endless filter belt passing therethrough. The pressure tank being cylindrically designed provides for a distribution of stresses so that it can be constructed with the smallest possible wall thickness. This structural concept also accommodates relatively simple manufacture of the pressure tank. The discharge openings are kept advantageously small and are in the form of rectangular openings for the filter belt requiring relatively low closing forces for the opening. Overall, the present concept accomplishes a light and simple execution and construction for a pneumatic pressure belt filter.
It has been discovered that the diameter of the cylindrical chamber is advantageously in the range of 50 to 150 cm, and more particularly, a diameter of 80 to 120 cm is preferred. The range between 50 and 150 cm is particularly advantageous for the working pressures of 2 through 6 bar regarding the weight per cm.sup.2 of filter surface whereby the range of size between 80 and 120 cm produces an optimization concerning the pressures, the overall weight of the device, and the filter surface. A diameter of 1 m and a working pressure of 4 bar has been found to be useful in operation with the compressed air provided from normal compressed air sources available in shops.
A further feature of the invention is that the filter belt is designed as a perforated metal endless belt. Filter belts heretofore used incorporated fabric, whether synthetic or metal, and required a high closing pressure to close the exposed cross-section areas existing between the individual threads of a synthetic or fabric belt.
With a perforated metal tape, a blocked cross-section advantageously exists between the individual holes and the closing force to be exerted can be considerably reduced.
A further feature of the invention is that the metal belt is constructed with zones without openings for filtered passage with the spacing between such zones corresponding to the spacing of the sealing devices, that is, the spacing between the openings to the cylindrical chamber. It is thus possible to advantageously seal the belt on a completely smooth metal surface at such zones without openings. This provides a problem-free seal which can be controlled. Further, an easy and thorough cleaning of the sealing area is possible. The only requirement is that the total length of the tape and its zones without opening correspond to a multiple of the spacing of the sealing devices.
A further feature of the invention is that the lower half of the cylindrical pressure tank contains fill equipment providing support and guide and collecting devices for the filtrate. A particularly favorable support for the belt with a uniform distribution of the pneumatic filtering forces on the pressure tank is thus obtained. The bridge-type structure for supporting the forces which are involved with higher pressures which were necessary in structures heretofore available is eliminated.
A further feature of the invention is that needle slot or bar screen is positioned on the upper side of the filler material which is located in the lower half of the cylindrical chamber. This needle slot screen provides substantial surface seating for the filter belt, but obtains unimpeded passages for the filtrate into the filtrate guide and collecting devices in the lower half of the chamber.
A further feature of the invention is the provision of pillow-like sealing devices at the rectangular openings leading to and from the chamber, and the pillow-like devices preferably are constructed of soft rubber with a steel backing carrier.
An advantage obtained from the pillow-like design of the sealing devices is that their surfaces do not lose their function even with surface irregularities occurring. The soft rubber adapts to the irregularities. Grains or the like which have not been completely removed are not disruptive since sufficient sealing surface for a faultless sealing is available due to the pillow seal shape. The steel belt on its steel support coacts with the pillow seal providing stability in operation.
The pillow-like sealing devices are constructed with an inside lip. This inside lip coacts with the steel filter belt improving the sealing effect with increasing pressure so that the sealing devices can be constructed lightweight. The lip is constructed with a width no greater than necessary so that it can be easily withdrawn or pivoted out of the filter cake on the belt.
A further feature of the invention resides in that the filter belt is looped or endless and is supported on spaced transport and deflection rolls which are driven by internally disposed motors which are preferably brake motors. A particularly compact structure of the belt drive is thus obtained with the use of brake motors, and the roll support obtains a precise positioning of the filter belt relative to the sealing zones.
In accordance with a preferred construction, the transport and deflection rolls are movable so that they can be raised and lowered. With this construction, the deflection rolls can lift the filter belt after termination of the filter operation so that the friction between the filter belt and its support is considerably reduced or essentially eliminated. This also increases the useful life of the filter belt. The raising and lowering of the filter belt can be attained by means of separate lifting elements, but also can be attained by using the torque in the startup of the belt transport.
In a preferred construction, the filter belt is designed as a woven steel belt with uniform smooth tape edges. A good lateral guidance and seal is enabled by the uniform tape edges to thereby further improve the filter operation.
In a preferred construction, a lapping belt is laid over the filter belt with the lapping belt formed of a fabric or other material, and this construction is frequently employed for the filtration of materials other than coal such as when used in the food industry. The fabric which is laid over the filter belt need not be endless but may be fed down onto the top of the filter belt from a supply roller.
A further feature of the invention is that the sealing devices are supported on lever actuator or carrying devices so that the motion of the sealing devices relative to the filter belt is at an angle of 20.degree. to 45.degree., and more preferably between 30.degree. and 40.degree. relative to a vertical line normal to the plane of the belt. This direction of movement of the sealing devices is particularly favorable for the pressure between the filter belt and the sealing surfaces and accomplishes a reliable fault-free seal of the area.
A further feature of the structure is that the pressure tank is divided by a horizontal partition plate below the filter belt and needle slot screen to provide a working space and a support space so that the support space under the partition plate is filled with fluid for a pressure balancing force.
A stable support for the filter belt and the underlying or supporting needle slot screen is attained by the aforedescribed construction. The support thus utilizes the concepts of air pressure and is not constructed so as to provide entire support by mechanical strength so that the support can be easily removed for repair or maintenance by bleeding out the pressure support fluid beneath the support. Ease of accessibility to the support space is thus assured, and uniform all-around pressure distribution against the inside walls of the cylindrical pressure tank is achieved by utilizing pressurized air as a support beneath the belt as well as the medium for forcing liquid from the coal or other material supported on the filter belt.
In accordance with the foregoing construction, the support space is interconnected with the space above the belt by a compensation pressure pipe having a pressure level indicator connected to the compressed air intake line for the workspace. In other words, the workspace pressure above the belt is related by a predetermined relationship to the support space below the support beneath the belt. The support space is provided with an aeration valve and a compensation pipe whereby a predetermined uniform pressure can be exerted and maintained. The level display in the pressure compensation pipe enables simple monitoring of the level of support fluid below the support plate.
In one form, the filter mechanism is operated automatically by an automatic timer control so that the filtering functions can be cycled and the mechanism be operated independent of manual control.
It is contemplated that a plurality of individual filters may be combined to form a filter unit to better exploit installation space such as by disposing the filters one above another such as when a battery of more than three filters is employed to achieve a continuous overall filter operation with only a slight pulsation or lack of uniformity of the constant throughput. With this arrangement, the continuity of output differs only inconsequentially from known disk or drum pressure filters.
Other objects, advantages and features, as well as equivalent methods and structures which are intended to be covered herein, will become more apparent from the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: